1. Field of the Invention
The present invention relates to co-channel interference filtering for digital television signals, and more particularly to methods and apparatus for implementing a co-channel interference filter.
2. Description of the Related Art
Conventional broadcast television signals are strictly analog in nature. These conventional signals generally conform to one of three broadcast formats in wide adoption: the NTSC (National Television Standards Committee) format adopted in the United States and a few other countries, and the PAL (Phase Alternation by Line) and SECAM (Systeme Electronique Couleur Avec Memoire) formats adopted in most other countries.
High-Definition Television (HDTV), or more generally Digital Television (DTV), formats abandon the conventional analog television signal format in favor of a digitally coded signal. Due to the high redundancy found in most video signals, it is possible to digitally compress a video sequence in a manner that will be visually imperceptible (or mostly so) once uncompressed. Such DTV signals can therefore transmit much more detail than is possible with an equivalent analog signal of the same bandwidth. With the current HDTV format being implemented in the United States, HDTV bandwidth has been set to occupy roughly the same bandwidth as an analog NTSC broadcast, with channels assigned from the same channel space as NTSC channels.
Although the long-term plan is to phase out NTSC channels, the vast majority of television users do not yet own HDTV receivers and a complete switchover does not appear imminent. In the interim, television stations that broadcast an HDTV signal may have viewers that receive both the desired HDTV signal and a relatively strong but undesired NTSC signal on the same channel. In this circumstance, the NTSC and HDTV signals interfere with each other, producing what is known as “co-channel” interference.
Referring to FIG. 1, a frequency spectrum 100 of interfering HDTV and NTSC signals is depicted. Envelope 110 represents the HDTV information transmitted within the NTSC signal spectrum. NTSC Video carrier V, located 1.25 MHz from the lower edge of the allotted frequency spectrum, is used to demodulate the luminance component of the original NTSC signal. Color subcarrier C, located 3.58 MHz above video carrier V, is used to demodulate the quadrature chrominance signals in an NTSC color television receiver. Audio carrier A, located 4.5 MHz above the video carrier, is used to demodulate the frequency-modulated (FM) NTSC audio signal transmitted in a relatively small frequency band centered about carrier A. Other NTSC signal energy of a considerably lower magnitude is also distributed throughout the illustrated frequency space.
When an HDTV signal occupies the same channel space as an NTSC signal, the NTSC signal can produce strong interference. It is therefore desirable to pre-filter the received HDTV signal with an NTSC rejection filter that can remove predictable components of the NTSC signal, i.e., the video, color, and audio carriers. Typically, a comb filter is used as the NTSC rejection filter. As shown in FIG. 1, the comb filter 120 has nulls spaced 57 fH Hz apart, where fH is the horizontal scan frequency of the analog video signal (15.734 kHz for NTSC video). One comb filter null aligns approximately with the video carrier V, another comb filter null aligns approximately with the color subcarrier C, and a third comb filter null aligns approximately with the audio carrier A.
From FIG. 1, it can be appreciated that the comb filter nulls are relatively wide, and the comb filter contains other nulls within the HDTV channel space that in all likelihood will not improve co-channel interference. In fact, the NTSC rejection filter degrades the signal-to-noise ratio (SNR) of the HDTV signal by approximately 3 dB when no NTSC signal is present. Therefore, an NTSC rejection filter that effectively filters NTSC carrier energy, without overly reducing the HDTV signal energy, would be preferred if such a filter could be implemented without undue complexity.
U.S. Pat. No. 5,325,188, entitled “Apparatus For NTSC Signal Interference Cancellation Through the Use of Digital Recursive Notch Filters” and issued to Scarpa, describes one type of filter for eliminating interfering V, C, and A NTSC carriers from an HDTV signal. As shown in FIG. 2, this patent describes an HDTV receiver 200 that uses separate digital recursive notch filters, i.e. a bi-quadratic filters, for each carrier component. An HDTV signal with NTSC interference is received at tuner 202, which rejects out-of-band signals and downconverts the desired signal to an intermediate frequency (IF). An analog-to-digital converter (ADC) 204 digitizes the IF signal, and supplies the digitized signal to one input of an adder 206 and to three bi-quadratic filters 210, 220, and 230. Each bi-quadratic filter filters all components of the input signal except for a narrow band around a frequency that the filter is tracking. The outputs of filters 210, 220, and 230 are subtracted from the digitized IF signal at adder 206, and the adder output is supplied to HDTV demodulator 240 for further processing.
Referring to graph 300 shown in FIG. 3, line 310 represents the intended frequency response observed at the output of adder 206. Each of filters 210, 220, and 230 produces a respective notch at one of the V, C, and A frequencies of potential NTSC interference. Although this response characteristic is considerably more selective to NTSC carriers than the comb filter frequency response characteristic 120 shown in FIG. 1, the bi-quadratic filters involve substantial computational complexity. Furthermore, each filter relies on an ability to obtain phase lock on an NTSC carrier of interest, which may be difficult when NTSC interference exists but does not contain a particularly strong carrier signal.
U.S. Pat. No. 6,219,088, entitled “NTSC Interference Rejection Filter” and issued to Liu et al., describes a different approach to NTSC carrier cancellation. The '088 patent describes an in-line NTSC filter 400 as shown in FIG. 4. Filter 400 uses three serial filter stages 410, 420, and 430 to respectively cancel NTSC video, color, and audio carrier signals. Each stage contains a frequency shifter (412, 422, and 432) and a DC cancel circuit (414, 424, and 434). Frequency shifter 412 shifts a basebanded HDTV signal spectrum to place the video carrier V at DC, and then DC cancel circuit 414 cancels that component. The output of DC cancel circuit 414 is supplied to frequency shifter 422, which shifts the signal spectrum to place the color subcarrier C at DC. DC cancel circuit 524 then cancels the color subcarrier component and supplies its output to frequency shifter 432. Frequency shifter 432 shifts the signal spectrum to place the audio carrier at DC, and then DC cancel circuit 534 cancels the audio carrier component. Finally, the frequency shifter 440 takes the output of DC cancel circuit 434 and removes the previous three shifts, thereby restoring the signal back to baseband.
Filter 400 is problematic in several respects. First, the desired components of the input signal pass through four frequency shifters and three filters, thereby adding magnitude and phase errors to the signal. Also, this filter does not account for lower sideband NTSC signal energy that is restored when the input signal is basebanded, and does not cancel this energy. The repeated frequency shifts by less than the width of the signal spectrum also shift the HDTV sidebands in an interfering manner, thus scrambling the desired HDTV signal.